DIAGENESIS AND FORMATION WATER CHEMISTRY OF TRIASSIC RESERVOIR SANDSTONES FROM SOUTHERN TUNISIA

The fluvial Triassic reservoir subarkoses and arkoses (2409.5-2519.45 m) of the El Borma oilfield, southern Tunisia, were subjected to cementation by haematite, anatase, infiltrated clays, kaolinite and K-feldspar at shallow burial depths from meteoric waters. Subsequently, basinal brines controlled the diagenetic evolution of the sandstones and resulted initially in the precipitation of quartz overgrowths, magnesian siderite, minor ferroan magnesite and anhydrite. The enrichment of siderite in C-12 isotope (delta(13)C(PDB) = -14.5 to -9 parts per thousand) results from derivation of carbon from the thermal decarboxylation of organic matter. During further burial, the precipitation of dickite and pervasive transformation of kaolinite into dickite occurred, followed by the formation of microcrystalline K-feldspar and quartz, chlorite and illite, prior to the emplacement of oil. Present day formation waters are Na-Ca-Cl brines evolved by the evaporation of seawater and water/mineral interaction and are in equilibrium with the deep burial (less than or equal to 3.1 km) minerals. These waters are suggested to be derived from the underlying Silurian and Devonian dolomitic mudstones.